Onions belong to the lily family, Amaryllidaceae, and the genus, Allium. Alliums comprise a group of perennial herbs having bulbous, onion-scented underground leaves, including such commonly cultivated crops as garlic, chives, and shallots. It also includes ornamental species grown for their flowers.
Onions are an important vegetable world-wide, ranking second among all vegetables in economic importance with an estimated value of $6 billion dollars annually. The onion is also one of the oldest cultivated vegetables in history. The common garden onions are in the species Allium cepa. Onions are classified in numerous ways, by basic use, flavor, color, shape of the bulb, and day length. Onions come in white, yellow, and red colors. The bulb may be rounded, flattened, or torpedo shaped.
Commercial onions include “storage onions”, “fresh onions”, “pearl or mini onions”, and “green onions”. “Fresh onions” tend to have a lighter color with a thin skin, a milder, sweeter flavor, and must be eaten fresh as they do not store well. These onions are available in red, yellow, and white colors, and are often sold under the name of their region, e.g., Sweet Imperials, Vidalias, Walla Walla Sweets and Texas Sweets. Perhaps the best known of the fresh onions is the Bermuda onion. Fresh onions are available beginning in March or early April and can be purchased until August.
Storage onions are available from harvest, which is at the beginning of August, and are stored and available throughout the winter months up to about March. Storage onions have a darker skin that is thicker than that of a fresh onion. They are also known for intense, pungent flavor, higher percentage of solids and desirable cooking characteristics. These onions are also available in red, yellow and white colors. Not all long day length type (long day type) onions are suitable for storage. A true storage onion is one that can be harvested in late summer or fall, and stored, under proper conditions, until the spring, when the fresh onion crop is again available.
“Spanish onion”, “Spanish onions”, or “Spanish type” are terms applied to various long-day onions, generally yellow, though some white, and generally varieties that are large and globe-shaped. Spanish onion is commonly applied to various long day type onions of the type grown in western states of the United States (California, Idaho, Oregon, Washington, Colorado) with a bulb size averaging 300-700 grams (g) (typically over 3 inches up to 4 inches but also up to 5 inches in diameter for bulbs classified as “colossal”).
Onion varieties initiate bulbing when both the temperature and a minimum number of daylight hours reach certain levels. When onions are first planted, they initially develop their vegetative growth, with no sign of bulb formation until the proper day length for that onion variety triggers the signal to the plant to stop producing above ground vegetative growth and start forming a bulb. Onions are thus sensitive to the hours of daylight and darkness they receive, and for most varieties it is only when the specific combination of daylight and darkness is reached, that the bulb starts to form. Onions are therefore classified by the degree of day length that will initiate bulb formation. Onions are described as short-, intermediate-, and long-day length types. Short day means that bulbing will initiate at 11 to 12 hours of daylight. Intermediate day is used for onions bulbing at 12 to about 14 hours of daylight. Long day onions require about 14 or more hours of daylight for bulb formation to start.
Growers producing onions in more northerly climates plant long-day length onions. Daylight length varies greatly with latitude, and at higher latitudes long-day onions will produce sufficient top growth before the day length triggers bulbing to produce a large bulb. A short-day onion grown in the North (higher latitudes) will bulb too early and produce relatively small bulbs.
Short day onions are preferred for southern areas such as southern Texas, southern California and Mexico. If a long day type onion is planted in such a short day climate, it may never experience enough day length to trigger the bulbing process.
Onions are also classified on flavor, with the common designations of sweet, mild, and pungent. The flavor of the onion is a result of both the type of onion and the growing conditions. For instance, soils containing a high amount of sulfur result in more pungent flavored onions. Sweetness in onions is caused by the sugars glucose, fructose and sucrose. Onions also contain polymers of fructose called fructans. Onion cultivars differ quite markedly in the relative amounts of sucrose, glucose, fructose and fructans which they contain. They also differ in sugars according to length of storage and location in the bulb. Short day cultivars, which are poor storers, tend to have higher levels of sucrose, fructose and glucose, but hardly any of the fructans. In contrast, long day type cultivars and intermediate storage cultivars such as Pukekohe Longkeeper have less sucrose, glucose and fructose and higher amounts of fructans.
The fructans do not play a role in sweetness. The balance between levels of pungency and levels of sugars determines the perception of pungency in an onion. High levels of pungency can mask high levels of sugars so that the onion is not perceived as sweet. Onions with low levels of pungency but low levels of sugars can be perceived as bland. Ideally a low pungency onion would have high levels of sugars and lower levels of pungency.
It is believed that sunlight strongly influences the development of pungent flavors. While compounds such as sugars and organic acids contribute to the flavor of onions, it is a special class of biologically active organosulfur compounds which give onions their distinctive flavor and aroma. Pungency in onions is caused by these volatile sulfur compounds, some of which affect the eyes when onions are fast cut and induce tearing (often called lachrymatory effect). There are 3 different flavor precursors in onions: 1-propenyl cysteine sulfoxide, which is usually found in the highest concentration; methyl-cysteine sulfoxide, which is normally found in lesser concentration; and propyl cysteine sulfoxide, which is found in the lowest concentration.
Storage conditions may also affect pungency, and though the research is conflicting, most studies show an increase in pungency for most long day type onions during storage. See Shock, C. C., E. B. G. Feibert, and L. D. Saunders. 2004. Pungency of Selected Onion Varieties Before and After Storage. Oregon State University, Malheur Experiment Station Special Report 1055: 45-46.
Within intact cells the enzyme allinase is compartmentalized in the cell vacuole and the flavor precursors are found in the cytoplasm. A reaction, therefore, only occurs when onion tissues are damaged and the enzyme and substrate are brought together as organelles are disrupted. The kinetics of decomposition are different for each specific flavor precursor. The decomposition of 1-propenyl cysteine sulfoxide is almost instantaneous, while the methyl and propyl cysteine sulfoxide decomposition occurs in several minutes. Primary products produced from flavor precursor decomposition include pyruvate, ammonia and chemically unstable sulfenic acids. Among the sulfenic acids is the lachrymator, or tear producing compound, characteristic of onions. The sulfenic acids undergo further rearrangement to form thiosulfinates, which are responsible for the characteristic flavor of onions.
Flavor precursor formation begins with the uptake of sulfate (SO4−2) by the onion, its reduction to sulfide, and subsequent assimilation into cysteine by light-dependent reactions in the leaves of the plant. Glutathione, a tripeptide of cysteine is then synthesized. This the starting point of the flavor precursor biosynthetic pathway. The pathways leading to the synthesis of each flavor precursor are not fully understood, although sulfur is known to be transformed through several identifiable peptide intermediates, each unique to a specific flavor precursor.
Researchers have recently developed a tool for documenting differences among onion flavor using a laboratory analysis of pyruvic acid development (PAD). Pyruvic acid has been shown to correlate well with consumer flavor perception. The PAD measurements are gaining acceptance within the industry as a clearer index of onion mildness, even though pungency is assessed solely by the amount of enzymatic pyruvic acid. For most commercial onions, pyruvic acid levels fall between about 1 and about 18 micromoles per a gram fresh weight. PAD units are given in micromoles pyruvic acid per gram, fresh weight (μmol/g FW). Short day onions marketed as low pungency onions will typically have PAD values of 5.5 μmol/g FW or less. Onion bulbs having a PAD of 5.5 μmol/g FW or less are considered sweet according to Vidalia Labs sweet onion certification specifications (Shock, C. C., E. B. G. Feibert, and L. D. Saunders. 2004. Onion Production from Transplants in the Treasure Valley. Oregon State University, Malheur Experiment Station Special Report 1055: 4752).
Long day onions are generally grown in the northern states, because of their requirement for long days to initiate bulb production. For this reason, long day type storage varieties do extremely well in the northern states of the United States and Canada, regions that have the required 14-16 hours of day-length during the summer. There are no long day type onions that have low pungency. Some of the commonly planted long day type yellow onion cultivars are Daytona, Ranchero, Granero, Sabroso, Tamara, Hamlet, Fortress, Norstar, Teton, and Vaquero.
Short day varieties do not keep well in storage conditions, and the pungency of short day varieties can climb considerably during storage. Present production in North America and Europe allows harvest of short day onions from mild winter regions from November through April. Long day onions are available fresh in the late summer and as storage onions from September through March, or even year round, have not been available in low pungency varieties. Sweet onions must be imported from the southern hemisphere to fill the gap in sweet onion production (November-February). In the United States, regions like Georgia and Texas produce short day onions from March to June, while low pungency onions available from November to February are short day onions, produced in the southern hemisphere.
There is a need in the art for the development of onions having low pungency, particularly, long-day varieties having low pungency. There is also a need for storage onions in which pungency does not substantially increase during storage.